Hard disk drives for portable applications are described at http://www.almaden.ibm.com/almaden/mobile_hard_drives.html.
An example of state-of-the-art power management for battery-operated systems is described in http://www.broadcom.com/products/Cellular/Power-Management-Solutions/BCM59001).
The disclosures of any publications and patent documents mentioned in the specification, and of the publications and patent documents cited therein directly or indirectly, are hereby incorporated by reference.